\documentclass{article} 
\usepackage{makecell}

\begin{document}
\begin{table}[htbp]
	\caption{Assumed transmission capacity for the Balmorel model.}
	\centering
	\begin{tabular}{l|c|c|c|c}
		\hline
		\multicolumn{2}{c|}{}                    & \multicolumn{2}{c|}{\thead{\textbf{Capacity [MW]}}} & \textbf{}           \\
		\hline
		\textbf{Technology type} & \textbf{Fuel} & \textbf{Spain} & \textbf{Portugal}                  & \textbf{Efficiency} \\
		\hline
		PT $\rightarrow$ ES      & 2954          & 3500           & 3500                               & 5\%                 \\
		ES $\rightarrow$ PT      & 2085          & 4200           & 4200                               & 5\%                 \\
		\hline
		FR $\rightarrow$ ES      & 450           & 2800           & 8000                               & 5\%                 \\
		ES $\rightarrow$ FR      & 325           & 2500           & 8000                               & 5\%                 \\
		\hline
	\end{tabular}
	\label{tab:transcap}
\end{table}
\end{document}
